A number of chromatographic techniques are known for separating the components of a sample. In packed column chromatography, a mobile phase fluid and a sample flow through a column containing a stationary phase, which is chosen to retain specific components of the sample. Supercritical fluid chromatography (SFC) uses a supercritical fluid, usually carbon dioxide, as the mobile phase. The solvent power of the mobile phase in supercritical fluid chromatography is a linear function of density, which is in turn related to pressure. However, in systems such as SFC systems that use compressible fluids at pressures above atmospheric (ambient), the pressure of the system is coupled to the mass flow rate.
The problems caused by the coupling between pressure and mass flow rate in supercritical fluid systems and their solution are explained in detail in commonly-assigned U.S. Pat. Nos. 5,133,859; 5,094,741 and U.S. patent application Ser. No. 804,155 filed Dec. 6, 1991--Frank et al., which are all incorporated herein by reference. Frank et al. teach that mass flow rate and pressure can be "decoupled" and independently controlled by providing a variable orifice restrictor downstream from, e.g., the column of a chromatographic instrument or the extraction chamber of an extraction instrument. Variable orifice restrictors and their use are disclosed in commonly-assigned U.S. Pat. Nos. 5,009,778; 5,151,178 and 5,178,767--Nickerson et al. which are all incorporated herein by reference. As used herein, "variable restrictors" refer to variable orifice restrictors such as those disclosed in the Frank and Nickerson patents, as well as other types of restrictors in which the degree of restriction is changed by varying the size of an orifice, occluding an orifice, changing other physical characteristics of an orifice or interchanging and selecting fixed orifices of varying sizes within a valve or similar device.
A specific application of supercritical fluid chromatography is the analysis of the aromatic content of diesel fuels using packed columns and a flame ionization detector (FID). The test method is set forth in ASTM D518691, published by the American Society for Testing and Materials, 1916 Race Street, Philadelphia, Pa. (USA) 19103-1187, which is incorporated herein by reference. The above-referenced ASTM method is generally applicable to many different types of petroleum and chemical samples where the goal is to determine the relative amounts of different classes of hydrocarbons. Basically, saturated hydrocarbons are separated from aromatic hydrocarbons in an SFC column and the flame ionization detector provides a signal based on the mass of carbon in each class of hydrocarbon.
The ASTM method specifies a fixed restrictor at the end of the SFC column to maintain the necessary pressure for supercritical fluid chromatography. However, there are several disadvantages to this configuration. First, fixed restrictors clog, changing retention time and the response of the flame ionization detector. Also, since every restrictor is different, no two sets of analysis equipment will have equivalent retention times, thereby introducing an uncertainty into the test method. Additionally, as explained above, in a fixed restrictor system, flow rate through the column is directly related to the pressure and the restrictor, so optimization and maintenance of chromatographic conditions is very difficult.
Another problem with the ASTM method is that the response of flame ionization detectors becomes nonlinear and non-uniform for the different classes of hydrocarbons as the flow the supercritical fluid mobile phase becomes too large. This limits the size of columns that can be used. Typically, the larger the column size, the larger the flow to the flame ionization detectors. Thus, in this type of system, smaller columns are less problematic than larger columns. It would be desirable to be able to control the flow rate to permit the use of columns having a larger inner diameter than typical SFC columns.
Finally, even though the flame ionization detector provides a signal representing the mass of the carbon atoms in the molecule, some diesel fuels with relatively the same amount of aromatic compounds perform quite differently in engines. Therefore, the flame ionization detector signal alone is not sufficient to differentiate all the diesel fuels being tested. Accordingly, it would be further desirable to provide a system wherein additional analytical techniques or instruments can be incorporated to simultaneously analyze the effluent stream from the SFC column to more accurately characterize the sample.